Lyme disease is caused by the tick borne spirochete Borrelia burgdorferi. The bacteria possess outer surface lipoproteins possessing a tripalmitoyl-S-glycerylcysteine (Pam3Cys) modification to the amino terminal cysteine with potent pro-inflammatory potential. The signaling receptor for Borrelia lipoproteins has been identified as toll-like receptor 2, TLR2, a member of a family of molecules involved in innate responses to microorganisms. Members of this family, including the IL-1 receptor, signal through a well characterized pathway of signaling molecules, resulting in NF-kB translocation to the nucleus and transcriptional activation of numerous pro-inflammatory genes. The consequences of direct activation of this extensive inflammatory pathway during infection has profound implications for our understanding of disease process and host defense to this organisms. This application proposes to assess the involvement of TLR2 mediated signaling by lipoproteins in the host-pathogen interactions of Lyme disease. In Aim 1, the involvement of TLR2 in B. burgdorferi induced arthritis will be determined by infecting mice possessing a null allele for tlr2. Arthritis severity and composition of inflammatory infiltrate at various time points following infection will be assessed. Parameters of host defense will also be determined in these mice, by assessing levels of spirochete DNA in tissues at different time points following infection, and assessing the kinetics of appearance of Borrelia specific antibodies of various isotypes. In the second Aim, the contribution of TLR2-independent interactions to the host response to B. burgdorferi infection will be assessed. The bacterial component responsible for lipoprotein-independent responses will be characterized and it will be determined if the response requires the TLR/IL-1R signaling pathway. In Aim 3, antibody reagents will be developed to mouse TLR2, and used to identify responsive cells in ankle and heart tissues. The direct involvement of TLR2-lipoproteins in pathological development will be studied by determining the extent of pathology attributable to TLR2 bearing cells in inflamed tissues. In the fourth Aim, the structural requirements for TLR2 interaction with bacterial lipoproteins will be studied, using mutant constructs that display altered responsiveness. Demonstration of direct interaction between lipoproteins and TLR2 will be sought, as will the involvement of partners in cellular responses to the lipoproteins. All experiments are focused on characterization of the host-pathogen interactions involved in pathological consequences of Lyme disease.